In an automatic transmission, depending on the operating mode selected, such as “forward drive”, “reverse drive”, “neutral” or “parking”, shift elements made as clutches or brakes are actuated by a hydraulic control system. Recent automatic transmissions often have an electrified hydraulic shift mechanism, by means of which a so-termed E-shift, i.e., an electrical active connection can be established between an operating device inside the vehicle and an electric hydraulic control device on the gearbox of the motor vehicle, without mechanical devices such as a selector slide plate.
An electronic-hydraulic device for an automatic shift motor vehicle transmission, in which the active connection between a device for influencing the shift and/or drive program and the transmission control is made electrically and in which, via the electric active connection, an electronic control unit of the electronic hydraulic control device is controlled directly, is described in DE 198 58 540 A1, to the full content of which reference is made. In the automatic transmission known therefrom, electrically controlled magnetic valves and electrically controlled pressure regulation valves are manipulated directly by the electronic control unit. If the electronic control unit fails, the magnetic valves provided as actuators and the electric pressure regulation valves are switched off. Thereby, by the action of pressure on certain clutches associated control valves are pressurized, so that driving can continue in an emergency gear.
If the vehicle is in a low gear before the defect occurs, then a low emergency gear is engaged. When the vehicle is in a higher gear before the defect occurs, a high emergency gear is engaged.
In an automatic transmission of the type with electrical control, the clutches must only be supplied with pressure after ensuring that the electronic transmission control system is fully operational. This supply takes place, as a rule, by moving a slide valve, usually known as the position valve, by means of an electric actuator.
To maintain the pressure supply in a transmission emergency program in which the electric supply has failed and all the electric actuators are in their starting position, the electric signal from the actuator must be replaced by another signal available in the emergency program since, otherwise, at least the most recently engaged operating condition or transmission range cannot be maintained.
Particularly in control devices with short hydraulic lines, which cannot provide any hydraulic elasticity because of the small volumes involved, spring feeds are used by virtue of which, if the electronic system fails, the pressure in the line can be maintained at least for as long as it takes for the spring feed to become unstressed. However, as soon as such a spring feed has become empty or the spring has reached an abutment, in this case too the pressure supply to the clutch collapses. Thus, such spring feeds are only able to bridge over very short electronic breakdowns, for example, 20 milliseconds or so, during which otherwise the valve would otherwise be switched over.
As a hydraulic spring feed the pressure of a clutch can be used whose pressure is maintained even during a brief undersupply of the system pressure. For example, if a pressure reservoir function of 1 bar has been chosen in the design of the position valve, the holding function must not exceed a clutch pressure of 1 bar.
The result of this low holding pressure is that in the “neutral” and “reverse drive” operating conditions, in some circumstances even when the control pressure is only very slightly deficient the self-holding of the position valve is activated. Thus, in the “neutral” operating condition in which all the clutch valves are pressurized but do not transfer the pressures to the clutches, a slight error on the part of the electronic transmission control or actuator leads to energizing of the valve which results in a pressure supply to a clutch. When the clutch has already reached its holding pressure for example, 1 bar when the fault is recognized, in an emergency program the shift position “neutral” actually desired, which was the operating condition before the defect occurred, can no longer be maintained. Rather, the vehicle drives on in an emergency gear, for example, third gear. Accordingly in some circumstances, the vehicle can begin moving even though the driver has selected the “neutral” shift position and so assumes that the vehicle will remain at rest.
A similar problem arises in the event of a fault when the operating condition “reverse drive” is engaged since, in this case, during emergency drive operation the operating condition “neutral” or “parking” should be selected and starting off, for example, in the third gear of the forward drive mode, should be avoided.
The purpose of the invention, therefore, is to provide a device for the control of a hydraulically actuated clutch of an automatic transmission of a motor vehicle, with which, by virtue of appropriate pressure regulator design, a current operating condition can be maintained until a sure fault diagnosis can be carried out by an electronic transmission control system, before an emergency gear is activated.